Many processes for the manufacture of alcohols produce an intermediate stream comprising an aqueous alcohol solution. This solution must be fractionated in order to produce the final alcohol product.
The accepted practice for the fractionation of alcohol solutions is to use distillation. While effective in this application, distillation has several drawbacks. A primary disadvantage of distillation is that it consumes considerable amounts of energy. Thus, as much as 50 to 80 percent of the energy required to produce ethanol in a typical fermentation process is consumed in the distillation step.
An added problem in producing absolute alcohol is that an azeotrope or constant boiling solution is formed during distillation. Ethanol, for example, produces an azeotropic composition of 95 weight percent alcohol and the balance water. Isopropyl alcohol has a binary azeotrope of 91 weight percent alcohol, the rest being water.
To circumvent the limitations posed by azeotropes, several creative procedures have been used. The classic method of producing anhydrous ethanol is to use benzene to break the azeotrope. This approach, however, introduces a toxic impurity. A newer technique is to resort to the use of molecular sieves to dehydrate the alcohol.